Iridescent Clouds Over Nova Scotia
March 07, 2003
Iridescent or opalescent clouds aren't actually a type of cloud but rather a type of light phenomenon that results from diffraction. These pastel colored clouds (sometimes referred to as irisation) are usually easier to discern with polarized sun glasses but are often vivid without any glare prevention. They can be thought of as detached coronae (see yesterday's Earth Science Picture of the Day), and they may be observed very near the Sun or as far away as 45 degrees from the Sun. It's thought that some of the colors (particularly the green shades) may be produced by especially minute water droplets in the clouds - just after the droplets have formed or are on the verge of dissipation (evaporation). A number of different clouds may display irisation, but when altocumulus or altostratus clouds are in the vicinity of the Sun, there's a better chance to see this phenomenon. The above photo was taken near Dartmouth, Nova Scotia on February 3, 2003 and was captured using a Kodak digital camera with ultraviolet and polarizing filters.